The present invention relates generally to wide-band pulse-compression radar systems and, more particularly, to pulse-compression code generators for use within such systems to generate a plurality of orthogonal codes which suppress jamming.
Significant advantages can be gained by using wide band pulse radars employing large pulse compression ratios. Such radars permit smaller range resolution cells due to the wider bandwidth. The decreased volume of these smaller range cells allows less chaff dipoles or raindrops per cell. Accordingly, such wide band radars are not blinded by backscatter or clutter energy reflections from rain or chaff. Additionally, such wide band radars are difficult to jam from long range because of the large power required to exceed the increased thermal noise power (due to the wider bandwidth) at the radar receiver.
However, two radars using the same frequency space and code will interfere with each other, i.e., the pulse transmissions from one radar will be compressed and processed by the other radar's compression system. In order to permit many wide band radars to share the same spectral space without mutual interference, multiple sets of uncorrelated codes are required, i.e., the cross-correlation coefficient for the codes must be some very small value such that the matched filter for one pulse radar will not compress the echoes from the transmissions of other radars into one range resolution cell. Unfortunately, the only known coding technique that will permit orthogonal codes to be developed, i.e., pseudorandom phase coding, is intolerant of doppler shift and thus cannot be used with targets with different velocities because the doppler shift will scramble the phase coding.
In this respect, the most doppler tolerant pulse coding sequence is linear FM or step approximation to linear FM pulse coding. This doppler tolerance is due to the fact that any doppler shift on an echo will simply translate all of the frequency components of the pulse by approximately the same amount in the same direction. Accordingly, all of the frequencies within the radar pass band will still exit from the matched filter at the same time relative to each other to form a short pulse. This output pulse will then occur at an absolute time different from that which would have resulted in the absence of doppler, i.e., range doppler coupling. However, this range-doppler coupling characteristic causes linear FM coded pulses to be extremely susceptible to radar jamming. Such jamming is accomplished by repeating a radar pulse transmission back to the radar at a later point in time with a frequency offset. Such a frequency outset signal will have the same slope as the transmitted signal but it will compress at the radar receiver to indicate a target with a range offset from the true target by the percentage of the uncompressed pulse length equal to the percentage of the frequency offset. If the offset is "up" in frequency, then a false target will be generated at a range either greater than or less than the true target range depending on whether an "up" chirp or a "down" chirp pulse is used. Likewise, if the offset is "down" in frequency, then a false target will be generated at a range either less than or greater than the true target range depending on whether an "up" chirp or a "down" chirp pulse is used. Thus, an enemy repeater could create one or more apparent or false targets both in front of and behind the true skin return by using frequency offsets.